The present invention relates to a pretensioner for vehicle seat belt systems, and in particular to a pretensioner having a return-prevention device for preventing the seat belt from losing part of its pretension after the pretensioner has operated.
In some seat belt systems used in vehicles such as automobiles, a pretensioner is provided for preventing movement of the body of an occupant through tightening of the belt when excessive acceleration acts on the vehicle and thereby providing greater restraint of the occupant. Pretensioners often have a return-prevention device for maintaining the tension applied to the belt by the pretensioner. Examples of pretensioners having return-prevention devices are found in Japanese Laid-open Patent Publications Nos. 61-241234, 1-119455, 1-164651, and 2-256550. All of those publications describe and show return-prevention devices in which a pawl associated with a belt-pulling member to which the belt is coupled is biased by a pawl spring in a direction to engage one-way (ratchet) locking teeth in the form of a rack. The spring urges the pawl into engagement with a tooth of the rack when the belt-pulling member starts to move in a direction that would allow the release of some of the tension in the belt. The engagement of the pawl with a ratchet tooth stops the belt-pulling member from moving further in a tension-releasing direction.
Seat belt pretensioners are required to operate very rapidly in order to tighten the belt before the occupant begins to move forward by the inertia imposed on him or her by the rapid vehicle acceleration (negative acceleration). Likewise, the pawl of a return-prevention device must engage a ratchet tooth in a very short time, lest a part of the pretension be lost by movement of the belt-pulling member in a direction that allows the belt to loosen. The movement of the pawl into engagement with a ratchet tooth of the rack requires that the inertia of the pawl be overcome. Hence, it would be desirable to provide a pawl having a low mass. Unfortunately, it is not feasible to reduce the mass of the pawl without also reducing its strength. The need for the pawl to be strong enough to support the load applied to the belt by the occupant places a lower limit on the mass of the pawl.